The invention generally relates to robotic systems and relates in particular to articulated arms that include end effectors that provide a vacuum source for object acquisition or gripping.
Such vacuum grippers exist in many configurations in the prior art. Generally, such devices use compressed air to generate a vacuum by use of a Venturi pump. The vacuum is then presented at the object to be acquired through any one of a variety of interfaces.
One type of interface is a single large open port, which maximizes the suction pressure from the vacuum and is thus well equipped to acquire heavy objects or those packaged in loose plastic. This configuration is commonly referred to as a bag type gripper.
Another type of interface is an array of smaller ports, each of which may have integrated flow control (due to their small hole size) designed to close or reduce them if they are not making contact with the object to be acquired. By closing unsealed ports, the suction pressure at ports that have successfully mated with the object to be acquired should be maximized. This approach provides flexibility in object acquisition since not all ports need to mate with an object in order to successfully acquire it. This flow control is generally accomplished by means of metering (or making the ports small enough that the resulting leakage from an unsealed port is immaterial).
In other vacuum gripper systems, integrated check valves may be used in port chambers that contact the environment. Typically, such devices include seals around the vacuum ports at the surface that meets the object to be acquired. This approach, while being more mechanically complicated, has the advantage of stronger overall suction force, since unsealed ports truly close at the opening, rather than just restricting leakage flow. For single large ports, a large suction cup or foam ring is used. For the array of ports configuration, an array of suction cups or a foam pad with holes for each of the individual ports are commonly used.
In some vacuum gripper systems, a set of actuated valves is provided for the air flowing through the gripper; a valve on the compressed air input allows for shutting the vacuum on and off, allowing the gripper to drop an object. This approach, however, is slow due to time constants of air pressure equalization within the gripper body. The speed of release is dramatically increased by adding a second controlled valve to the exhaust port of the vacuum generator; by closing this valve, compressed air is diverted through the gripper body and out the vacuum ports, effectively blowing the acquired object off of the gripper surface quickly.
The vacuum grippers in the prior art are generally designed for a specific object or material in a predetermined orientation. The specific gripper style and configuration is chosen to optimize for a particular acquisition problem (for instance, palletizing and de-palletizing a particular size/type of cardboard cartons). Such grippers are not at all well suited to a wide array of objects in non-predetermined orientations.
Further, in such vacuum gripper systems, software systems and algorithms are provided that are developed around the concept of maximizing speed and efficiency of abort/retry cycles for robotic manipulators acquiring objects. These algorithms have been focused in some applications on heavily instrumented multiple-finger mechanical grasping manipulators. The algorithms use the data from joint angles and motor power to determine how well an object is grasped and immediately retry if the grasp is not good enough.
In many such vacuum gripper systems, abort/retry techniques with vacuum grippers are unsophisticated. These techniques generally consist of applying vacuum, lifting the gripper, and looking at coarse flow rate or weight sensors to determine whether an object has been acquired; if it has not, the gripper is placed back on the object and acquisition is re-attempted. This is due largely to two reasons: 1) most currently deployed vacuum gripping systems are customized so heavily for the material being acquired that acquisition failures are rare, resulting in no real need for rapid abort and retry cycles, and 2) no vacuum grippers exist with the type of sophisticated instrumentation present in, for instance, a multi-fingered grasping type end effector. The result is that existing abort/retry algorithms cannot obtain from a vacuum gripper the information they need to be able to operate.
There remains a need, therefore, for an improved vacuum gripper for use in an articulated arm that provides improved performance in acquiring a wide variety of known and unknown objects.